1. Field of the Invention
The present invention relates to a fixing apparatus and an image formation apparatus.
2. Description of the Related Art
A fixing apparatus is used by an image formation apparatus that is included in, e.g., a copying machine, a printer, a facsimile apparatus, or a compound machine that has at least two of the above functions. The fixing apparatus, which is a thermal fixing apparatus, uses either a heat roller fixing method that provides a high thermal efficiency and safety using a contact-type heating roller, or a film heating method using an energy-saving type film.
The thermal fixing apparatus using the heat roller fixing method includes a heating roller that rotates and is for heating (fixing roller), and an elastic pressurization roller that rotates and makes pressing contact with the fixing roller. The fixing roller and the pressurization roller form a pressing-contact nip (fixing nip). Through the nip, recording paper P that carries a yet-to-be fixed image (toner image), the recording paper P being the target of heating, is conveyed. Heat is applied by the fixing roller and pressure is applied at the nip to the toner image such that the toner image is permanently fixed on the recording paper P.
The heating roller of the thermal fixing apparatus generally includes a metal core having a predetermined thickness, and a layer of an elastic material or a mold-release material provided on the surface of the metal core. Further, a heat source, such as a halogen lamp, is arranged inside the heating roller so that the heating roller is heated from the interior. The metal core is usually made of aluminum because aluminum has excellent processing and thermal conductivity properties.
Recently and continuing, reduction of power consumption is strongly desired due to environmental concerns, while high definition and high speed image production are desired to meet market demand. Then, in order to meet the requirements, various improvements to the fixing apparatus of the heating roller method have been attempted.
[Patent reference 1] JPA 10-301417
[Patent reference 2] JPA 11-73050
[Patent reference 3] JPA 63-313182
[Patent reference 4] JPA 2-157878
[Patent reference 5] JPA 4-44075
[Patent reference 6] JPA 4-204980
One of the improvements is concerning the fixing roller, wherein shortening a temperature rising time and reducing power consumption of the fixing roller are aimed at. There, a heat source is arranged for heating outside surfaces of the fixing roller and the pressurization roller, where only the outside surfaces of the rollers are heated such that the fixing apparatus requires only low power and provides high thermal efficiency. Examples of this improvement are disclosed by Patent References 1 and 2. According to the examples, the heat source is arranged contacting the outside surface of the fixing roller, or alternatively, is contactlessly arranged. Thermal propagation efficiency is higher if the heat source contacts the heating roller. Further, there are types such as one where the heat source is a halogen lamp arranged inside a hollow metal core, and another where an exothermic resistance layer is arranged inside the hollow metal core through an insulating layer of an organic resin (e.g., polyimide), or alternatively, of glass.
Another of the improvements is concerning the film heating method, wherein a heat-resistant film (a fixing film, or simply a film), which is rotationally driven, for heating is stuck to an exothermic body by a pressurization roller (an elastic roller), and sliding conveyance is carried out. Recording paper P (imprint paper) that carries a yet-to-be fixed image (toner image) and the fixing film are conveyed through a pressing-contact nip formed by the exothermic body (heater) and the pressurizing roller (pressurization member) so that the toner image is permanently fixed on the recording paper P by the heat from the exothermic body through the fixing film and the pressure applied at the nip. Examples are disclosed by Patent References 3 through 6.
An example of the film heating method is shown by FIG. 1A, wherein a heater 103 consisting of an exothermic resistance layer formed on a ceramic board made of such as alumina and aluminum nitride is fixed (arranged) to a stay holder (supporting member) 101. Further, a fixing film 100 that is heat-resistant and made of, e.g., polyimide is adhered to the heater 103. Further, a pressurization roller 110 including a metal core 111, an elastic layer 112, and a mold release layer 113 applies force to and contacts the heater 103 through the fixing film 100 to form a nip. The fixing film 100 is rotationally driven by rotation of the pressurization roller 110 and is heated by the heater 103 at the nip. Temperature of the heater 103 is detected by a temperature detecting unit (thermistor) 102 that is attached to the back of the heater 103, and is fed back to a control unit (not illustrated) so that the temperature of the heater 103 is controlled to be a predetermined temperature (fixing temperature). When used in a printer or a copying machine, an image formation apparatus that includes a fixing apparatus of the film heating method has advantages, such as dispensing with pre-heating while in standby and a shortened wait time, due to higher thermal efficiency and faster temperature rising rate than the conventional heat roller method.
Nevertheless, the conventional methods described above have the following problems.
First, according to the heat roller method, the fixing roller has a great heat capacity, and for this reason, a quick start is not available. Further, if an elastic layer, such as silicone rubber, is provided on the fixing roller surface for improvement in speed and quality, the heat capacity is increased and the elastic layer surface has to be heated from the interior of the fixing roller. Accordingly, there is a problem in that wait time is prolonged.
Then, a method of providing a quick start and high quality is proposed as shown in FIG. 1B. Here, an elastic roller using such as silicone rubber is used as a fixing roller 105, and image degradation is reduced while the heat transfer efficiency to the paper is improved by a wrapping effect of the elastic layer. Further, the thermal efficiency is raised by directly heating the surface of the fixing roller by a ceramic heater 107, which is conventionally used by the film heating method, and by using a thin film and a heat insulation backup member as a pressurization member 115. In this way, a quick start is made possible. However, according to the thermal fixing apparatus as described above, since the fixing roller uses the heat insulation elastic material, the heat capacity is less than that of the conventional the fixing roller used by the conventional fixing roller method, which poses a problem in that the fixing roller temperature greatly fluctuates while processing the recording paper. Further, since a heating nip for heating the fixing roller 105 is arranged at the perimeter of the fixing roller 105, there is a problem in that changing the temperature of the fixing roller 105 is delayed, unlike in the case of on-demand fixing where heating is carried out at the fixing nip. Further, it is desired that the pressurization roller 115 provide uniform temperature distribution in order to carry out stable fixing, and for this purpose a width greater than the width of the recording paper P is heated. This poses a problem in that additional power is required.
Next, according to the film heating method, since the thin film having small heat capacity is used and the fixing nip is intensively heated, a quick start is possible; however, temperature control of the fixing nip is difficult. Further, since the thin film is driven by the pressurization roller, film speed tends to be lower than the rotational speed of the pressurization roller, which poses a problem of degrading an image to be fixed.
Furthermore, there is also a problem called an “idle portion temperature rising” phenomenon with the fixing apparatus using the film heating method. That is, when an exothermic body has an “idle portion” where the paper does not pass when printing on small size (width) paper, the “idle portion” is excessively heated, given that the exothermic body almost uniformly generates heat to the maximum paper width of the image formation apparatus, and that the heat is not dissipated by the paper at the “idle portion”. Accordingly, when printing on small size paper such as an envelope, since the heat at the “idle portion” is not transferred to the paper, the temperature of the fixing apparatus is excessively raised. Although this phenomenon is also a problem with the fixing apparatus of the conventional heat roller fixing method, the phenomenon is more of a problem with the fixing apparatus of the film heating method. This is because, in order to provide on-demand service, the heat capacity of the exothermic body such as the heating film is made small, and the “idle portion temperature rising” phenomenon notably appears. Therefore, the temperature of the “idle portion” becomes considerably higher, compared to a portion where the paper is processed, after fixing a great amount of small size paper, such as envelopes. If the paper of a usual size (width being near the maximum paper width) is processed under this condition, since the temperature of the “idle portion” is excessively high, “hot offset” is produced in the portion corresponding to the small paper “idle portion”, degrading the printing quality.
Conventionally, this problem is solved by reducing throughput, that is, reducing the frequency of printing by extending the paper feed interval. However, the on-demand fixing apparatus of the film heating method is required to provide high speed printing; accordingly, lowering the throughput in order to solve the problem of “idle portion temperature rising” is not a desired solution.
Another method to solve the problem is providing two or more exothermic bodies corresponding to sizes (widths) of the paper to be printed. However, it is conventionally impossible to prepare exothermic bodies corresponding to a great number of paper sizes, or to provide a thermistor to each part.
Further, since heat capacity of the fixing apparatus using the film heating method is small, the temperature has to be finely controlled according to the temperature of the pressurization roller. For example, when the thermal fixing apparatus is started from room temperature, the temperature of the pressurization roller is low; therefore, a greater amount of heat has to be provided. To the contrary, if the temperature of the pressurization roller is high because of immediately preceding fixing operations, heat of the pressurization roller can be transferred to the recording paper P; accordingly, the amount of heat to be provided may be small. For this reason, the fixing apparatus of heating film type includes a thermistor for measuring the temperature of the fixing apparatus so that a proper amount of heat is provided, the thermistor being attached to the exothermic body. The thermistor is often arranged at a place where the paper of any allowed size (width) passes. Accordingly, when small size paper is passed, temperature rising at the “idle portion” is not properly detected. That is, the temperature detected by the thermistor is considerably different from the actual temperature of the fixing apparatus. Further, even if standard size paper is processed, both ends of the exothermic body tend to be cooled faster than central portions; therefore, there is often a difference between the temperature detected by the thermistor and the actual fixing temperature. As described above, if the amount of heat to be provided is determined only by the temperature detected by the thermistor at the paper passing portion, hot offset and poor fixing (cold offset) may be produced.